1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a reflective type liquid crystal display device for displaying images using light incident from an observation side, and a semi-transmissive type liquid crystal display device for displaying images by selectively or simultaneously using transmitting light incident from a side opposite to the observation side and light incident from the observation side.
2. Description of the Related Art
The liquid crystal display device is thin, light-weighted and exhibits the low power consumption and hence, the liquid crystal display device has been used as a display device in a wide range of electronic equipments including a notebook-type personal computer, a word processor, an electronic notebook, a mobile phone, a camera-built-in video recorder and the like. Unlike a cathode ray tube and a plasma display device, the liquid crystal display device is not self-luminescent and displays images or the like by controlling a quantity of light incident from the outside. Further, it is possible to perform a color image display of multi colors with the use of color filters in plural colors as light control elements.
This type of liquid crystal display device sandwiches a liquid crystal layer between a pair of substrates (hereinafter, referred to as “an upper substrate” and “a lower substrate”) and visualizes an electronic latent image by controlling the orientation of molecules of the liquid crystal composition which constitutes the liquid crystal layer in response to an electric field applied to the liquid crystal layer.
The liquid crystal display device is classified, in accordance with a driving method, into a simple matrix type liquid crystal display device and an active matrix type liquid crystal display device. The currently-available liquid crystal display device is capable of performing a high-definition and high-speed display and hence, the active matrix type liquid crystal display devices are mainly used. In the active matrix type liquid crystal display device, active elements (switching elements) which are represented by thin film transistors for pixel selection are provided to the above-mentioned lower substrate or upper substrate and color filters colored in three colors separately for color display are provided to either one of these substrates. In a reflective-type liquid crystal display device, images are displayed by using light incident from an observation side, while in a semi-transmissive type liquid crystal display device, images are displayed by selectively or simultaneously using a transmitting light incident from a side opposite to the observation side and light incident from the observation side.
Since the liquid crystal display device is not a self-luminous type display device, it is necessary to visualize an electronic latent image with illumination composed of a visible light and to emit the visualized image as an image light to an observation surface. A method which emits the illumination light such as a natural light (an external light) or the like from the observation surface side is referred to as a reflective type, while a method which emits an illumination light from a side opposite to the observation surface is referred to as a transmissive type. Further, a liquid crystal display device which adopts both of the method which emits the illumination light from the observation surface side and the method which emits the illumination light from the side opposite to the observation surface is referred to as a semi-transmissive type (a semi-transmissive/reflective type). Here, a liquid crystal display cell which is formed into a semi-transmissive type by providing reflecting plates to a lower substrate and by forming openings in portions of the reflecting plates has been commercialized. As a document which discloses this type of art, JP-A-7-333598 (a patent document 1) can be named.
FIG. 13 is a schematic cross-sectional view of the vicinity of one pixel for explaining a constitutional example of the semi-transmissive type liquid crystal display device. A liquid crystal display device PNL is formed by stacking various types of optical members described later to a liquid crystal display cell LCD which is constituted of a lower substrate SUB1 preferably made of glass and having reflecting plates (reflecting electrodes) RF and transparent pixel electrodes ITO1 on an inner surface thereof and an upper substrate SUB2 having a transparent common electrode ITO2 on an inner surface thereof which faces the lower substrate SUB1 in an opposed manner and laminated to the lower substrate SUB1 with a liquid crystal layer LC sandwiched therebetween.
Here, thin film transistors are formed on the lower substrate SUB1 as active elements. Each thin film transistor is constituted of a gate electrode GT having an anodized film AO on a surface of aluminum and neodymium (Al—Nd), a gate insulation film GI made of silicon nitride (SiN), a silicon semiconductor film Si, a source electrode SD1 and a drain electrode SD2. The pixel electrode ITO1 which is formed of a transparent electrode is connected to the source electrode SD1. A passivation film PAS which is formed of an insulation film is formed such that the passivation film PAS covers the source electrodes SD1 and the drain electrodes SD2, while the reflecting electrodes RF are formed over the passivation film PAS. The reflecting electrode RF is connected to the source electrode SD1 via a contact hole CH formed in the passivation film PAS in a penetrating manner. The reflecting electrode RF has a function of a reflecting plate and a function of a pixel electrode.
An opening ST which is formed by cutting off the reflecting electrode RF is provided to a portion of the reflecting electrode RF thus forming a semi-transmissive reflection film which allows light incident from outside (lower side in FIG. 13) of the lower substrate SUB1 to pass through the liquid crystal layer LC toward the upper substrate SUB2 side. In the drawing, a symbol Cadd indicates an additional capacitance of the pixel and generates a given capacitance using the passivation film PAS provided between the electrode which is formed as a film simultaneously with the gate electrode GT and the reflecting electrode RF as a dielectric layer. Further, to an uppermost layer which is brought into contact with the liquid crystal layer LC, a lower orientation film ORI1 is applied and the orientation treatment in a given direction is applied to the lower orientation film ORI1. Although there may be a liquid crystal display device which is provided with a leveling film below the lower orientation film ORI1, the leveling film is not shown in the drawing here. Following optical members are stacked on upper and lower surfaces of the liquid crystal display cell LCD. Here, spacers which define a distance between upper and lower substrates are omitted from FIG. 13.
First of all, on an outer surface of the lower substrate SUB1 of the liquid crystal display cell LCD, a lower λ/4 phase difference plate PSQ1, a lower λ/2 phase difference plate PSH1 and a lower polarizer POL1 are stacked in this order. On the other hand, over an inner surface of the upper substrate SUB2, color filters CF of three colors (R, G, B) which are defined by a black matrix BM are formed. The color filters CF are covered with a leveling film OC2 and the common electrode ITO2 is further formed over the leveling film OC2 (only one color filter shown in FIG. 13). Then, to an uppermost layer which is brought into contact with the liquid crystal layer LC, an upper orientation film ORI2 is applied and the orientation treatment in a.given direction is applied to the upper orientation film ORI2. In this constitutional example, an opening HL which enhances the brightness by directly emitting a reflecting light from the reflecting electrode RF to the upper substrate SUB2 is formed in a portion of the color filter CF. However, the opening HL is not an inevitable constitutional feature. On an outer surface (observation side) of the upper substrate SUB2, an upper λ/4 phase difference plate PSQ2, an upper λ/2 phase difference plate PSH2 and an upper polarizer POL2 are stacked in this order. Here, the upper λ/4 phase difference plate PSQ2 is adhered to the upper substrate SUB2 using a diffusion tacky adhesive layer SC.
FIG. 14 is a developed view for specifically explaining one example of the stacked structure of respective optical members in the liquid crystal display device shown in FIG. 13. On the upper side (observation side) of the semi-transmissive type liquid crystal display cell LCD, the upper λ/4 phase difference plate PSQ2, the upper λ/2 phase difference plate PSH2 and the upper polarizer POL2 are stacked in this order. Further, on the lower side of the liquid crystal display cell LCD, the lower λ/4 phase difference plate PSQ1, the lower λ/2 phase difference plate PSH1 and the lower polarizer POL1 are stacked in this order thus constituting the liquid crystal display device PNL as a whole.